Tydex - FTIR Beam Splitter
FTIR spectrometer is generally based on Michelson interferometer where one of the mirrors is movable. Two mirrors are located at both arms of interferometer and oriented perpendicularly to each other. Beamsplitter is placed at the vertex of the right angle and oriented at a 45° angle relative to each mirror. Light passing to the beamsplitter is divided into two parts (50%/50% ideally) those propagate further to the both arms and are reflected off the mirrors. The beam which is one time reflected from the beamsplitter (upper beam on the drawing) must also pass there and back through an inclined compensator plate to compensate for the fact that the other beam passes through the beamsplitter plate three times instead of one. The movable mirror is scanned over some distance that produces an interference pattern of the two beams reaching the detector. Fourier transform of the signal encodes the spectrum of a source (investigated material).
The wavelength range from visible to Far IR can be covered by choosing the appropriate material of beamsplitter and compensator. The material choice includes the following materials: fused silica (quartz) (Visible/Near-IR), CaF2, BaF2, and ZnSe (Near-IR/Mid-IR).High Resistivity FZ-Silicon could also be recommended for beamsplitters in Far IR region. Due to Fresnel reflection it can be used for ~50%/50% beam splitting in very wide wavelength range without any coating.
In order to achieve high resolution of the FTIR spectrometer the beamsplitter/compensator pairs should be produced with very high degree of accuracy. Particularly surface flatness, wedge tolerance, and thickness matching are together of great importance.